Degradation evaluation of microelectromechanical thermal actuators

Metal based thermal microactuators normally have lower operation temperatures than those of Si-based ones; hence they have great potential for applications. However, metal-based thermal actuators easily suffer from degradation such as plastic deformation. In this study, planar thermal actuators were made by a single mask process using electroplated nickel as the active material, and their thermal degradation has been studied. Electrical tests show that the Ni-based thermal actuators deliver a maximum displacement of ∼20μm at an average temperature of ∼420°C, much lower than that of Si-based microactuators. However, the displacement strongly depends on the frequency and peak voltage of the pulse applied. Back bending was clearly observed at a maximum temperature as low as 240°C. Both forward and backward displacements increase with increasing the temperature up to ∼450°C, and then decreases with power. Scanning electron microscopy observation clearly showed that Ni structure deforms and reflows at power above 50mW. The compressive stress is believed to be responsible for Ni piling-up (creep), while the tensile stress upon removing the pulse current is responsible for necking at the hottest section of the device. Energy dispersive X-ray diffraction analysis revealed severe oxidation of the Ni-structure induced by Joule-heating of the current.

Thermal modelling of a tubular linear machine for marine renewable generation

A lumped parameter thermal model has been constructed for a tubular linear machine that has been designed for use in a marine environment. It shows good correlation to both steady state and transient experimental tests on the machine. The model has been developed for a stationary machine in a laboratory environment - the modelling techniques used and enhancements to enable the application of the model directly to marine scenarios are discussed.

Thermal material with low curie temperature in a thermally actuated superconducting flux pump system

A thermally actuated flux pump is an efficient method to magnetize the high-temperature superconductor (HTS) bulk without applying a strong magnetic field. A thermal material is employed as a magnetic switch, which decides the efficiency of the system. To measure the Curie temperatures of those samples without destroying them, the nondestructive Curie temperature (NDT) measurement was developed. The Curie temperature of gadolinium (Gd) was measured by the NDT method and compared to the results from superconducting quantum interference device (SQUID). Because the SQUID tests require the sample to be cut into small piece, a constant shape of the testing sample could not be guaranteed. The demagnetizing effect was considered to remove the shape effect. The intrinsic permeability was modified from the apparent susceptibility by considering demagnetization. A thermal material with low Curie temperature, Mg 0.15Cu0.15Zn0.7Ti0.04Fe 1.96O4, was synthesized and its performance was tested and compared with previous thermal materials. Comparisons of three thermal materials, including the Curie temperature and the permeability, will be detailed in the paper. © 2002-2011 IEEE.

Detailed thermal modelling of a tubular linear machine for marine renewable generation

A detailed lumped-parameter thermal model is presented for a tubular linear machine that has been designed for use in a marine environment. The model has been developed for a static machine, the worst-case thermal scenario, and is used to establish a rating for the machine. The model has been validated against a large range of experimental tests and shows good correlation to both steady-state and transient experimental results. The model was constructed from a mostly theoretical basis with very little calibration, suggesting that the techniques used are applicable in a more general sense. © 2013 IEEE.

Behavior of RC beams retrofitted with CARDIFRC after thermal cycling

This study investigates the effect of thermal cycling on the performance of concrete beams retrofitted with CARDIFRC, a new class of high performance fiber-reinforced cement-based material that is compatible with concrete. Twenty four beams were subjected to 24 h thermal cycles between 25 and 90°C. One third of the beams were reinforced either in flexure only or in flexure and shear with conventional steel reinforcement and used as control specimens. The remaining sixteen beams were retrofitted with CARDIFRC strips to provide external flexural and/or shear strengthening. All beams were exposed to a varied number of 24 h thermal cycles ranging from 0 to 90 and were tested in four-point bending at room temperature. The tests indicated that the retrofitted members were stronger and stiffer than control beams, and more importantly, that their failure initiated in flexure without any signs of interfacial delamination cracking. The results of these tests are presented and compared to analytical predictions. The predictions show good correlation with the experimental results. © 2010 ASCE.

Nanometer scale thermal drift of 4Gbit/s uncooled laser for tight-channel-spacing low-cost WDM

A 4Gbit/s directly modulated DBR laser is demonstrated with nanometre scale thermal tuning over an extended 20-70°C temperature range. >40dB side mode suppression over the entire temperature range is achieved. © 2005 Optical Society of America.

Porous materials for thermal management under extreme conditions

A brief analysis is presented of how heat transfer takes place in porous materials of various types. The emphasis is on materials able to withstand extremes of temperature, gas pressure, irradiation, etc., i.e. metals and ceramics, rather than polymers. A primary aim is commonly to maximize either the thermal resistance (i.e. provide insulation) or the rate of thermal equilibration between the material and a fluid passing through it (i.e. to facilitate heat exchange). The main structural characteristics concern porosity (void content), anisotropy, pore connectivity and scale. The effect of scale is complex, since the permeability decreases as the structure is refined, but the interfacial area for fluid-solid heat exchange is, thereby, raised. The durability of the pore structure may also be an issue, with a possible disadvantage of finer scale structures being poor microstructural stability under service conditions. Finally, good mechanical properties may be required, since the development of thermal gradients, high fluid fluxes, etc. can generate substantial levels of stress. There are, thus, some complex interplays between service conditions, pore architecture/scale, fluid permeation characteristics, convective heat flow, thermal conduction and radiative heat transfer. Such interplays are illustrated with reference to three examples: (i) a thermal barrier coating in a gas turbine engine; (ii) a Space Shuttle tile; and (iii) a Stirling engine heat exchanger. Highly porous, permeable materials are often made by bonding fibres together into a network structure and much of the analysis presented here is oriented towards such materials. © 2005 The Royal Society.

Thermal transport in high porosity cellular metal foams

The heat dissipation capability of highly porous cellular metal foams with open cells subject to forced air convection is studied using a combined experimental and analytical approach. The cellular morphologies of six FeCrAlY (an iron-based alloy) foams and six copper alloy foams with a range of pore sizes and porosities are quantified with the scanning electronic microscope and image analysis. Experimental measurements on pressure drop and heat transfer for copper foams are carried out. A numerical model for forced convection across open-celled metal foams is subsequently developed, and the predictions are compared with those measured. Reasonably good agreement with test data is obtained, given the complexity of the cellular foam morphology and the associated momentum/energy transport. The results show that cell size has a more significant effect on the overall heat transfer than porosity. An optimal porosity is obtained based on the balance between pressure drop and overall heat transfer, which decreases as the Reynolds number is increased.

Measurements of thermal radiation in ultralight metal foams with open cells

Highly porous ultralightweight cellular metal foams with open cells have attractive mechanical, thermal, acoustic and other properties and are currently being exploited for high-temperature applications (e.g. acoustic liners for combustion chambers). In such circumstances, thermal radiation in the metal foam becomes a significant mechanism of heat transfer. This paper presents results from experimental measurements on radiative transfer in Fe-Cr-Al-Y (a steel-based high-temperature alloy) foams having high porosity (95 per cent) and different cell sizes, manufactured at low cost from the sintering route. The spectral transmittance and reflectance are measured at different infrared wavelengths ranging from 2.5 to 50 μm, which are subsequently used to determine the extinction coefficient and foam emissivity. The results show that the spectral quantities are strongly dependent on the wavelength, particularly in the short-wavelength regime (less than 25 μm). While the extinction coefficient decreases with increasing cell size, the effect of cell size on foam reflectance is not significant. When the temperature is increased, the total extinction coefficient increases but the total reflectance decreases. The effective radiative conductivity of the metal foam is obtained by using the guarded hot-plate apparatus. With the porosity fixed, the effective radiative conductivity increases with increasing cell size and increasing temperature. © IMechE 2004.